Oscillation in receive system?

[Posting2002]

Hi, I have a design of a satellite antenna and a low noise amplifier, and
have a problem after connecting them together...right now, the connection is
like this:

1) Antenna -> LNB -> Spectrum analyzer

2) Antenna -> LNA -> LNB -> Spectrum analyzer

The Antenna is 25 dB of gain, LNA is 20 dB of gain. Input match of the
antenna (SMA connection) is better than 10 dB, LNA S11 and S22 are better
than 10 dB as well. Noise figure of LNA is 1 dB. LNB downconverts to IF
and has 20 dB of gain by itself.

The results of the spectrum analyzer show some kind of oscillatory behavior
in the system.

http://www.ece.uwaterloo.ca/~dbusuioc/comparison2.jpg

With connection (1) I get the blue graph, with connection (2) I get the
green and red graphs (tested 2 different LNAs).

I am thinking there is a mismatch problem in the antenna -> LNA or LNA ->
LNB. I've tested with a loss pad between LNA and LNB to see if the
oscillatory behavior is gone and it's only reduced in power but still
present.

Anybody have any suggestion or ideas?

Thanks,



/Dan

 

[maxfoo]

Hi, I have a design of a satellite antenna and a low noise amplifier, and
have a problem after connecting them together...right now, the connection is
like this:

1) Antenna -> LNB -> Spectrum analyzer

2) Antenna -> LNA -> LNB -> Spectrum analyzer

The Antenna is 25 dB of gain, LNA is 20 dB of gain. Input match of the
antenna (SMA connection) is better than 10 dB, LNA S11 and S22 are better
than 10 dB as well. Noise figure of LNA is 1 dB. LNB downconverts to IF
and has 20 dB of gain by itself.

The results of the spectrum analyzer show some kind of oscillatory behavior
in the system.

http://www.ece.uwaterloo.ca/~dbusuioc/comparison2.jpg

With connection (1) I get the blue graph, with connection (2) I get the
green and red graphs (tested 2 different LNAs).

I am thinking there is a mismatch problem in the antenna -> LNA or LNA ->
LNB. I've tested with a loss pad between LNA and LNB to see if the
oscillatory behavior is gone and it's only reduced in power but still
present.

Anybody have any suggestion or ideas?

Thanks,



/Dan

could be the spec ana causing the problem...
try a circulator/ isolator 'tween the spec a and lnb.







Remove "HeadFromButt", before replying by email.

 

[Ken Taylor]

Hi, I have a design of a satellite antenna and a low noise amplifier, and
have a problem after connecting them together...right now, the connection is
like this:

1) Antenna -> LNB -> Spectrum analyzer

2) Antenna -> LNA -> LNB -> Spectrum analyzer

The Antenna is 25 dB of gain, LNA is 20 dB of gain. Input match of the
antenna (SMA connection) is better than 10 dB, LNA S11 and S22 are better
than 10 dB as well. Noise figure of LNA is 1 dB. LNB downconverts to IF
and has 20 dB of gain by itself.

The results of the spectrum analyzer show some kind of oscillatory behavior
in the system.

http://www.ece.uwaterloo.ca/~dbusuioc/comparison2.jpg

With connection (1) I get the blue graph, with connection (2) I get the
green and red graphs (tested 2 different LNAs).

I am thinking there is a mismatch problem in the antenna -> LNA or LNA ->
LNB. I've tested with a loss pad between LNA and LNB to see if the
oscillatory behavior is gone and it's only reduced in power but still
present.

Anybody have any suggestion or ideas?

Thanks,



/Dan

What sweep time were you using? Are you sure you're not just seeing 60Hz hum
from your power supply?

Ken

 

[Dan]

Hi, I have a design of a satellite antenna and a low noise amplifier, and

What sweep time were you using? Are you sure you're not just seeing 60Hz hum
from your power supply?

Ken

Sorry I don't have access to my other e-mail, so I can't post
there...The sweep time I use is 1 second for the 500 MHz band, with a
3 kHz resolution bandwidth. There is no power supply on the LNA I use
a DC battery to keep the power noise low.

Any other ideas? It clearly looks like oscillation to me, right?


Thanks,


/Dan

 

[Dan]

could be the spec ana causing the problem...
try a circulator/ isolator 'tween the spec a and lnb.

Well as you can see in the graph, with only the antenna, LNB, and
analyzer it recovers the spectrum successfully, so there's nothing
after the LNB, it's working properly. Should be something between
antenna and LNA or LNA and LNB.

Let me know if you can think of anything else.

Thanks,




/Dan

 

[Posting2002]

Hi, I have a design of a satellite antenna and a low noise amplifier, and

have a problem after connecting them together...right now, the connection is
like this:

1) Antenna -> LNB -> Spectrum analyzer

2) Antenna -> LNA -> LNB -> Spectrum analyzer

The Antenna is 25 dB of gain, LNA is 20 dB of gain. Input match of the
antenna (SMA connection) is better than 10 dB, LNA S11 and S22 are better
than 10 dB as well. Noise figure of LNA is 1 dB. LNB downconverts to IF
and has 20 dB of gain by itself.

The results of the spectrum analyzer show some kind of oscillatory behavior
in the system.

http://www.ece.uwaterloo.ca/~dbusuioc/comparison2.jpg

With connection (1) I get the blue graph, with connection (2) I get the
green and red graphs (tested 2 different LNAs).

I am thinking there is a mismatch problem in the antenna -> LNA or LNA ->
LNB. I've tested with a loss pad between LNA and LNB to see if the
oscillatory behavior is gone and it's only reduced in power but still
present.

Anybody have any suggestion or ideas?

Thanks,



/Dan

Found the solution; I played with very tiny pieces of absorbing material in
the first cavity of the low noise amplifier, and this changed the impedance
slightly and eliminated mismatch, so now I have no resonance and good
amplification.

Problem is that by doing so the noise figure of the amplifier is increasing
so I have higher losses, maybe it needs more tuning.



/Dan